An infrared study on the conformation of poly‐L‐proline in concentrated aqueous salt solutions

The infrared absorption of poly‐ L ‐proline in concentrated aqueous salt solutions was measured in the fundamental region. Of primary interest were the carbonyl absorption of the peptide linkage and the methylene C–H bending absorption of the pyrrolidine ring. These spectral regions each show an additional component in the concentrated salt solutions. Using the position of the absorptions of poly‐ L ‐proline I ( cis ) and II ( trans ) as models, we conclude that both cis – trans linkages are present in the peptide in salt solutions. Increasing the temperature shifts the equilibrium slightly in favor of cis .